Xenobiotic biotransformation in unicellular green algae. Involvement of cytochrome P450 in the activation and selectivity of the pyridazinone pro-herbicide metflurazon

The N-demethylation of the pyridazinone pro-herbicide metflurazon into norflurazon implies a toxification in photosynthetic organisms. This is confirmed by quantitative structure activity relationships determined for two unicellular green algae, Chlorella sorokiniana and Chlorella fusca; however, the latter is 25 to 80 times more sensitive to metflurazon. This sensitivity is linked to differences in the N-demethylase activity of both algae, as determined by an optimized in vivo biotransformation assay. Apparent K(m) values of the metflurazon-N-demethylase indicate a 10-fold higher affinity for this xenobiotic substrate for Chlorella fusca. Furthermore, algal metflurazon-N-demethylation is characterized by distinct variations in activity, depending on the stage of cell development within the cell cycle. Several well-established inhibitors of cytochrome P450-mediated reactions, including piperonylbutoxide, 1-aminobenzotriazole, 1-phenoxy-3-(1H-1,2,4-triol-1yl)-4-hydroxy-5,5-dimethylhexane++ +, and tetcyclacis, as well as cinnamic acid, a potential endogenous substrate, inhibited the N-demethylation of metflurazon. The results suggest that the N-demethylation of metflurazon by both algae is mediated by a cytochrome P450 monooxygenase. The determination of antigenic cross-reactivity of algal proteins with heterologous polyclonal antibodies originally raised against plant P450s, anti-cinnamic acid 4-hydroxylase (CYP73A1), anti-ethoxycoumarin-O-dealkylase, anti-tulip allene oxidase (CYP74), and an avocado P450 (CYP71A1) or those of bacterial origin, CYP105A1 and CYP105B1, suggests the presence of distinct P450 isoforms in both algae.

Characterization of hsr201 and hsr515, two tobacco genes preferentially expressed during the hypersensitive reaction provoked by phytopathogenic bacteria

During an incompatible interaction between tobacco and the bacterial phytopathogen Pseudomonas solanacearum, 2 classes of genes, the so-called hsr (hypersensitivity-related) genes, activated preferentially during the hypersensitive reaction, and the str (sensitivity-related) genes, expressed strongly during compatible and incompatible interactions, have been identified. In this report, two hsr cDNA clones, hsr515 and hsr201, as well as their expression patterns are presented. Hsr515 was found to encode a P450 monooxygenase and is most similar to the ripening-related avocado gene CYP71A1 (40.6% amino acid identity). Hsr201 presents 58.6% amino acid identity with pTom36, a tomato gene expressed during fruit maturation. The putative functions of the hsr gene products appear to be quite diverse and their characteristics of activation were found to be very conserved: accumulation of the corresponding mRNAs primarily in leaf areas in contact with the avirulent P. solanacearum strain or with a Pseudomonas fluorescens strain containing the hrpZ gene encoding a necrotizing polypeptide, harpin and absence of expression during normal plant development. Our results also suggest that, in a tobacco line expressing NahG, a lower level of salicylic acid, a compound associated with systemic acquired resistance, and also possibly involved in the development of necrotic lesions characteristic of the HR, does not affect the hsr gene expression.

Evaluation of Oxyrase enrichment method for isolation of Campylobacter jejuni from inoculated foods

Recovery limits were evaluated for Campylobacter jejuni in an existing Food and Drug Administration (FDA) enrichment broth (EB) formula supplemented with Oxyrase enzyme. Cultures of Camp. jejuni were inoculated into EB or EB containing 10% raw milk, raw oysters, crabmeat or mushrooms. After 24 and 48 h of enrichment, Camp. jejuni was isolated on four selective agars. No significant differences in recovery rates for Camp. jejuni were observed in the Oxyrase enrichment under normal atmosphere or in the existing FDA method under modified atmosphere. Increase of enrichment time from 24 to 48 h did not improve the recovery rates. However, the Oxyrase enrichment was cost effective, less time consuming, and simpler to perform than the established method.

Susceptibilities of 201 anaerobes to erythromycin, azithromycin, clarithromycin, and roxithromycin by oxyrase agar dilution and E test methodologies

The susceptibility of 201 anaerobes to erythromycin, azithromycin, clarithromycin, and roxithromycin was tested by agar dilution and E test methods by using a commercially available plate and dish system (OxyDish) to provide anaerobic conditions. Plates were incubated for 48 h. MICs for 50% of strains tested and MICs for 90% of strains tested by agar dilution and E test methods corresponded within 1 doubling dilution for all compounds. When all antibiotics were considered together, agar and E test MICs were within 1 and 2 doubling dilutions of each other in 84 to 91% and > 99% of cases, respectively.

Effect of CO2 on susceptibilities of anaerobes to erythromycin, azithromycin, clarithromycin, and roxithromycin

The Oxyrase agar dilution method (Oxyrase, Inc., Mansfield, Ohio), which provides an anaerobic environment without added CO2, was compared with the reference agar dilution method recommended by the National Committee for Clinical Laboratory Standards (anaerobic chamber with 10% CO2) to test the susceptibilities of 302 gram-negative and gram-positive anaerobes to erythromycin, azithromycin, clarithromycin, and roxithromycin. For erythromycin, the overall MIC for 50% of isolates tested (MIC50) was 0.5 micrograms/ml and the MIC90 was 8.0 micrograms/ml by the Oxyrase method, whereas they were 4.0 and 64.0 micrograms/ml, respectively, under standard anaerobic conditions with CO2. At a breakpoint of 4.0 micrograms/ml, 88% of strains were susceptible to erythromycin by the Oxyrase method, whereas 63% were susceptible in the chamber. The corresponding MIC50s and MIC90s of azithromycin, clarithromycin, and roxithromycin by the Oxyrase method were 0.5 and 8.0, 0.25 and 4.0, and 0.5 and 16.0 micrograms/ml, respectively, whereas in the chamber they were 4.0 and > 64.0, 2.0 and 64.0, and 2.0 and 64.0 micrograms/ml, respectively. At a breakpoint of 8.0 micrograms/ml for these three drugs, 89, 92, and 85% of the isolates, respectively, were susceptible by the Oxyrase method, whereas 67%, 72, and 68% of the isolates, respectively, were susceptible in the chamber. Most strains resistant to all four compounds by both methods were Bacteroides distasonis, Fusobacterium mortiferum, Fusobacterium varium and non-Clostridium perfringens Clostridium species. Results of the study may lead to a reappraisal of the role played by macrolides and azalides in the treatment of anaerobic infections.

Enzymatic determination of itoic acid, a Bacillus subtilis siderophore, and 2,3-dihydroxybenzoic acid

A specific enzymatic method to determine the amounts of itoic acid, a Bacillus subtilis siderophore, and 2,3-dihydroxybenzoic acid (2,3-DHBA) was devised. A sample was incubated first with hippurate hydrolase and then with 2,3-DHBA-3,4-dioxygenase. Itoic acid was estimated from the increase in A374. The incubation with the first enzyme was omitted for the determination of 2,3-DHBA.

Oxyrase, a method which avoids CO2 in the incubation atmosphere for anaerobic susceptibility testing of antibiotics affected by CO2

The Oxyrase agar dilution method, with exclusion of CO2 from the environment, was compared with the reference agar dilution method recommended by the National Committee for Clinical Laboratory Standards (anaerobic chamber with 10% CO2) to test the susceptibility of 51 gram-negative and 43 gram-positive anaerobes to azithromycin and erythromycin. With the Oxyrase method, anaerobiosis was achieved by incorporation of the O2-binding enzyme Oxyrase in addition to susceptibility test medium, antibiotic, and enzyme substrates into the upper level of a biplate. Plates were covered with a Brewer lid and incubated in ambient air. With azithromycin, Oxyrase yielded an MIC for 50% of strains tested (MIC50) and MIC90 of 2.0 and 8.0 micrograms/ml, compared to 8.0 and > 32.0 micrograms/ml in standard anaerobic conditions. At a breakpoint of 8.0 micrograms/ml, 90.4% of strains were susceptible to azithromycin with Oxyrase, compared to 53.2% in the chamber. The corresponding erythromycin MIC50 and MIC90 were 1.0 and 8.0 micrograms/ml with Oxyrase, compared to 4.0 and > 32.0 micrograms/ml by the reference method, with 89.3% of strains susceptible at a breakpoint of 4 micrograms/ml with Oxyrase, compared to 60.6% in CO2. Exclusion of CO2 from the anaerobic atmosphere when testing for susceptibility to azalides and macrolides yielded lower MICs, which may lead to a reconsideration of the role played by these compounds in treatment of infections caused by these strains.

Opposite facial specificity for two hydroquinone epoxidases: (3-si,4-re)-2,5-dihydroxyacetanilide epoxidase from Streptomyces LL-C10037 and (3-re,4-si)-2,5-dihydroxyacetanilide epoxidase from Streptomyces MPP 3051

(3-si,4-re)-2,5-Dihydroxyacetanilide epoxidase (DHAE I), a key enzyme in the biosynthesis of the epoxysemiquinone antibiotic LL-C10037 alpha by Streptomyces LL-C10037 [Gould, S.J., & Shen, B. (1991) J. Am. Chem. Soc. 113, 684-686], and (3-re,4-si)-2,5-dihydroxyacetanilide epoxidase (DHAE II) isolated from Streptomyces MPP 3051--which yields the (3R,4S)-epoxyquinone mirror image product of DHAE I--are described. DHAE I was purified 640-fold. Gel permeation chromatography indicated an Mr of 117,000 +/- 10,000; SDS-PAGE gave a major band of 22,300 daltons, indicating that DHAE I is either a pentamer or hexamer in solution. The enzyme had a pH optimum of 6.5, a Km of 8.4 +/- 0.5 microM, and a Vmax of 3.7 +/- 0.2 mumol min-1 mg-1. DHAE II was purified 1489-fold. The enzyme was shown to be a dimer of Mr 33,000 +/- 2000, with 16,000-dalton subunits, with a pH optimum of 5.5 and a Km of 7.2 +/- 0.4 microM. Both enzymes required only O2 and substrate; flavin and nicotinamide coenzymes had little or no effect. Neither catalase nor EDTA affected the activity of either enzyme, but complete inhibition of both was obtained with 1,10-phenanthroline. The activity of the purified DHAE I could be enhanced, but only by Mn2+ (relative V = 246 at 0.04 mM), Ni2+ (relative V = 266 at 0.2 mM), or Co2+ (relative = 498 at 0.2 mM). Reconstitution from a DHAE I apoenzyme, generated by treatment with 1,10-phenanthroline followed by Sephadex G-25 chromatography, occurred only by addition of one of these three metals.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel method of inducing and assuring total anoxia during in vitro studies of O2 deprivation injury

Oxyrase is an enzyme mixture coveted by microbiologists for its unique ability to remove O2 from media in which anaerobic bacteria are grown. The study reported here examined the potential usefulness of Oxyrase as an adjunct to gassing freshly isolated rat proximal tubules (RPT) with 95% N2-5% CO2 in an attempt to achieve totally O2-free conditions (anoxia) before initiating studies on the mechanism of O2 deprivation injury in vitro. RPT, in 6 ml of Krebs-Henseleit buffer (KHB), were initially gassed with 95% N2-5% CO2 at 1.5 liters/min for 5 min and incubated for 15 to 30 min at 37 degrees C in a shaking water bath, pO2 decreased from approximately 400 to 80 mm Hg. If RPT were present in the KHB, pO2 was even lower, i.e., approximately 50 mm Hg. Addition of increasing concentrations of Oxyrase (300 to 1,500 mU) to KHB alone, that is, without RPT, reduced pO2 from 80 mm Hg to less than 5 mm Hg; increasing the gas rate from 1.5 to 3.0 liter/min of 95% N2-5% CO2, the concentration of Oxyrase to 1,800 mU, and adding RPT reduced pO2 to zero. In this latter condition, pO2 remained unmeasurable during the 20 min of study and neither pH nor pCO2 changed compared with control values. Oxyrase (1,800 mU) had no effect on lactate dehydrogenase release, a sign of membrane injury, in normoxic RPT in KHB. We conclude that anoxia can easily be achieved by the addition of Oxyrase to KHB in which RPT are suspended, if the appropriate concentration of Oxyrase is added and if the RPT are gassed with 95% N2-5% CO2. This concentration of Oxyrase exerts no detrimental effects on RPT gassed with 95% O2-5% CO2.

Utilization of aromatic substances by Azotobacter chroococcum

Azotobacter chroococcum utilized catechol, protocatechuic acid and gentisic acid as sole carbon source. Rothera's test indicated the ortho cleavage of phenols. The cells displayed high levels of dioxygenase activity. Catechol was cleaved by the ortho pathway mediated by catechol 1,2-dioxygenase. Protocatechuate and gentisic acid were also cleaved by the ortho pathway and cells grown in protocatechuate and gentisic acid displayed protocatechuate, 3,4-dioxygenase and gentisate dioxygenase activity, respectively.

[Oxygenase activity of cytochrome P-450 electrochemically reduced in the presence of methylviologen]

An electrochemical system of cytochrome P-450 reduction in the presence of the water-soluble redox carrier methylviologen has been developed. In this system cytochrome P-450 effectuates a steady-state demethylation of dimethylaniline and hydroxylation of aniline. The results of control experiments suggest that the above reactions are mediated by cytochrome P-450. The effect of the peroxidase reaction is excluded by an addition of high concentrations of catalase to the incubation mixture. At the same time the hydroxylation of these substrates is accompanied by methylviologen demethylation.

X-ray absorption spectroscopy of the [2Fe-2S] Rieske cluster in Pseudomonas cepacia phthalate dioxygenase. Determination of core dimensions and iron ligation

We have employed X-ray absorption spectroscopy to obtain structural information about the Rieske Fe/S center in the phthalate dioxygenase (PDO) from Pseudomonas cepacia. Native PDO contains a dinuclear Rieske Fe/S center and an additional mononuclear Fe site. In order to study selectively the Fe/S cluster, we measured data for samples in which the mononuclear site was either depleted of metal or reconstituted with Co or Zn. Our results demonstrate that the iron environment in the Rieske cluster is structurally indistinguishable from that found in other Fe/S clusters, thus strongly supporting the suggestion that the unusually high reduction potentials for Rieske clusters are due to electrostatic rather than structural effects. The average Fe-Fe distance is 2.68 (3) A for both oxidized and reduced Rieske clusters. The average Fe-S distance is 2.24 (2) A in the oxidized cluster and 2.28 (2) A in the reduced cluster. Careful analysis of the EXAFS Debye-Waller factors suggests that the bridging and terminal Fe-S distances for the oxidized cluster are 2.20 and 2.31 A, respectively. Taken together with recent ENDOR results, these studies provide a detailed structural model for the Rieske [2Fe-2S] centers.

Confirmation of a ping-pong mechanism for S-adenosyl-L-methionine:magnesium protoporphyrin methyltransferase of etiolated wheat by an exchange reaction

An exchange reaction between unlabeled S-adenosyl-L-methionine and radiolabeled S-adenosyl-L-homocysteine has been used to confirm the occurrence of a ping-pong mechanism in S-adenosyl-L-methionine:magnesium protoporphyrin methyltransferase of etiolated wheat. The enzyme, S-adenosyl-L-homocysteine hydrolase, has been used to prepare radiolabeled S-adenosyl-L-homocysteine from labeled adenosine and DL-homocysteine. The exchange reaction was accomplished with a methyltransferase preparation purified by affinity chromatography on hemin-linked Sepharose 4B, and radioactivity was exchanged into unlabeled S-adenosyl-L-methionine to an extent of 70% of the theoretical maximum value.

Structure and assembly of protocatechuate 3,4-dioxygenase

Dioxygenases catalyse the cleavage of molecular oxygen with subsequent incorporation of both oxygen atoms into organic substrates. Some of the best-studied dioxygenases have been isolated from bacteria where they catalyse the critical ring-opening step in the biodegradation of aromatic compounds. These bacterial enzymes generally contain nonheme ferric iron as the sole cofactor. Protocatechuate 3,4-dioxygenase (3,4-PCD) was one of the first such enzymes recognized and catalyses the intradiol cleavage of protocatechuic acid by oxygen to produce beta-carboxy-cis,cis-muconic acid. Previous studies have shown that the 3,4-PCD found in Pseudomonas aeruginosa is an oligomer with a relative molecular mass (Mr) of 587,000 (587K) containing 12 copies each of alpha (22.3K) and beta (26.6K) subunits. The X-ray structure determination of 3,4-PCD reveals the catalytic iron environment required for oxygenolytic cleavage of aromatic rings and also provides a novel holoenzyme assembly with cubic 23(T) symmetry and first examples of mixed beta-barrel domains.

Probing structure-function relations in heme-containing oxygenases and peroxidases

Structural factors that influence functional properties are examined in the case of four heme enzymes: cytochrome P-450, chloroperoxidase, horseradish peroxidase, and secondary amine mono-oxygenase. The identity of the axial ligand, the nature of the heme environment, and the steric accessibility of the heme iron and heme edge combine to play major roles in determining the reactivity of each enzyme. The importance of synthetic porphyrin models in understanding the properties of the protein-free metal center is emphasized. The conclusions described herein have been derived from studies at the interface between biological and inorganic chemistry.

Determination of the quaternary structure of protocatechuate 3,4-dioxygenase from Pseudomonas aeruginosa

A 2.5 A resolution data set has been collected for crystals of protocatechuate 3,4-dioxygenase from Pseudomonas aeruginosa. Analysis of the data using the rotation function shows that the alpha 2 beta 2 tetramers associate to form a particle with cubic 23 (T) point group symmetry. Prior to this analysis it was believed that eight tetramers associated to form the holoenzyme. The symmetry of the crystalline holoenzyme also addresses questions concerning its iron content and substrate stoichiometry.

Enzymatic determination of veratryl alcohol

A rapid and sensitive method was developed for the measurement of veratryl alcohol--a secondary metabolite of some lignin degrading fungi. The method is based on the enzymatic oxidation of veratryl alcohol to veratraldehyde by the ligninase of Phanerochaete chrysosporium. The purified enzymes oxidized veratryl alcohol completely to veratraldehyde (75%) and some unidentified products. The enzymatic method was applied to measure veratryl alcohol in the culture filtrates of Chrysosporium pruinosum and it gave the same results as the conventional method involving extraction and separation by high-pressure liquid chromatography. Benefits and limitations of the method are discussed.